131 research outputs found
Dynamic criticality far-from-equilibrium: one-loop flow of Burgers-Kardar-Parisi-Zhang systems with broken Galilean invariance
Burgers-Kardar-Parisi-Zhang (KPZ) scaling has recently (re-) surfaced in a
variety of physical contexts, ranging from anharmonic chains to quantum systems
such as open superfluids, in which a variety of random forces may be
encountered and/or engineered. Motivated by these developments, we here provide
a generalization of the KPZ universality class to situations with long-ranged
temporal correlations in the noise, which purposefully break the Galilean
invariance that is central to the conventional KPZ solution. We compute the
phase diagram and critical exponents of the KPZ equation with -noise
(KPZ) in spatial dimensions using the dynamic
renormalization group with a frequency cutoff technique in a one-loop
truncation. Distinct features of KPZ are: (i) a generically
scale-invariant, rough phase at high noise levels that violates
fluctuation-dissipation relations and exhibits hyperthermal statistics {\it
even in d=1}, (ii) a fine-tuned roughening transition at which the flow
fulfills an emergent thermal-like fluctuation-dissipation relation, that
separates the rough phase from (iii) a {\it massive phase} in (in
the interface is always rough). We point out potential connections to
nonlinear hydrodynamics with a reduced set of conservation laws and noisy
quantum liquids.Comment: 29 pages, 11 figures, 1 table, 54 references, v2 as publishe
Fluctuations of imbalanced fermionic superfluids in two dimensions induce continuous quantum phase transitions and non-Fermi liquid behavior
We study the nature of superfluid pairing in imbalanced Fermi mixtures in two
spatial dimensions. We present evidence that the combined effect of Fermi
surface mismatch and order parameter fluctuations of the superfluid condensate
can lead to continuous quantum phase transitions from a normal Fermi mixture to
an intermediate Sarma-Liu-Wilczek superfluid with two gapless Fermi surfaces --
even when mean-field theory (incorrectly) predicts a first order transition to
a phase-separated "Bardeen-Cooper-Schrieffer plus excess fermions" ground
state. We propose a mechanism for non-Fermi liquid behavior from repeated
scattering processes between the two Fermi surfaces and fluctuating Cooper
pairs. Prospects for experimental observation with ultracold atoms are
discussed.Comment: as accepted to Phys. Rev. X; 10 pages, 10 figures, 75 reference
Umklapp Superradiance from a Collisionless Quantum Degenerate Fermi Gas
The quantum dynamics of the electromagnetic light mode of an optical cavity
filled with a coherently driven Fermi gas of ultracold atoms strongly depends
on geometry of the Fermi surface. Superradiant light generation and
self-organization of the atoms can be achieved at low pumping threshold due to
resonant atom-photon Umklapp processes, where the fermions are scattered from
one side of the Fermi surface to the other by exchanging photon momenta. The
cavity spectrum exhibits sidebands, that, despite strong atom-light coupling
and cavity decay, retain narrow linewidth, due to absorptionless transparency
windows outside the atomic particle-hole continuum and the suppression of
inhomogeneous broadening and thermal fluctuations in the collisionless Fermi
gas.Comment: Revised version, as accepted to Physical Review Letter
Continuois Time Contests
This paper introduces a contest model in which each player decides when to stop a privately observed Brownian motion with drift and incurs costs depending on his stopping time. The player who stops his process at the highest value wins a prize. Applications of the model include procurement contests and competitions for grants. We prove existence and uniqueness of the Nash equilibrium outcome, even if players have to choose bounded stopping times. We derive the equilibrium distribution in closed form. If the noise vanishes, the equilibrium outcome converges to - and thus selects - the symmetric equilibrium outcome of an all-pay auction. For two players and constant costs, each player’s profits increase if costs for both players increase, variance increases, or drift decreases. Intuitively, patience becomes a more important factor for contest success, which reduces informational rents
Gambling in Contests
This paper presents a strategic model of risk-taking behavior in contests. Formally, we analyze an n-player winner-take-all contest in which each player decides when to stop a privately observed Brownian Motion with drift. A player whose process reaches zero has to stop. The player with the highest stopping point wins. Contrary to the explicit cost for a higher stopping time in a war of attrition, here, higher stopping times are riskier, because players can go bankrupt. We derive a closed-form solution of the unique Nash equilibrium outcome of the game. In equilibrium, the trade-off between risk and reward causes a non-monotonicity: highest expected losses occur if the process decreases only slightly in expectation
Universality in antiferromagnetic strange metals
We propose a theory of metals at the spin-density wave quantum critical point
in spatial dimension . We provide a first estimate of the full set of
critical exponents (dynamical exponent , correlation length , spin susceptibility , electronic non-Fermi liquid
, spin-wave Landau damping ), which
determine the universal power-laws in thermodynamics and response functions in
the quantum-critical regime relevant for experiments in heavy-fermion systems
and iron pnictides. We present approximate numerical and analytical solutions
of Polchinski-Wetterich type flow equations with soft frequency regulators for
an effective action of electrons coupled to spin-wave bosons. Performing the
renormalization group in frequency -instead of momentum- space allows to track
changes of the Fermi surface shape and to capture Landau damping during the
flow. The technique is easily generalizable from models retaining only patches
of the Fermi surface to full, compact Fermi surfaces.Comment: 46 pages, 13 figures, typos fixed; as accepted to Physical Review
Dual QED3 at "NF = 1/2" is an interacting CFT in the infrared
We study the fate of weakly coupled dual QED3 in the infrared, that is, a
single two-component Dirac fermion coupled to an emergent U(1) gauge field, but
without Chern-Simons term. This theory has recently been proposed as a dual
description of 2D surfaces of certain topological insulators. Using the
renormalization group, we find that the interplay of gauge fluctuations with
generated interactions in the four-fermi sector stabilizes an interacting
conformal field theory (CFT) with finite four-fermi coupling in the infrared.
The emergence of this CFT is due to cancellations in the -function of
the four-fermi coupling special to "NF = 1/2". We also quantify how a possible
"strong" Dirac fermion duality between a free Dirac cone and dual QED3 would
constrain the universal constants of the topological current correlator of the
latter.Comment: 21 pages, 8 figures; v2 minor typos fixe
FFLO strange metal and quantum criticality in two dimensions: theory and application to organic superconductors
Increasing the spin imbalance in superconductors can spatially modulate the
gap by forming Cooper pairs with finite momentum. For large imbalances compared
to the Fermi energy, the inhomogeneous FFLO superconductor ultimately becomes a
normal metal. There is mounting experimental evidence for this scenario in 2D
organic superconductors in large in-plane magnetic fields; this is complemented
by ongoing efforts to realize this scenario in coupled tubes of atomic Fermi
gases with spin imbalance. Yet, a theory for the phase transition from a metal
to an FFLO superconductor has not been developed so far and the universality
class has remained unknown. Here we propose and analyze a spin imbalance driven
quantum critical point between a 2D metal and an FFLO phase in anisotropic
electron systems. We derive the effective action for electrons and bosonic FFLO
pairs at this quantum phase transition. Using this action, we predict non-Fermi
liquid behavior and the absence of quasi-particles at a discrete set of hot
spots on the Fermi surfaces. This results in strange power-laws in
thermodynamics and response functions, which are testable with existing
experimental set-ups on 2D organic superconductors and may also serve as
signatures of the elusive FFLO phase itself. The proposed universality class is
distinct from previously known quantum critical metals and, because its
critical fluctuations appear already in the pairing channel, a promising
candidate for naked metallic quantum criticality over extended temperature
ranges.Comment: 3+1 figure
- …